Technological development and increased demand for mobile equipment have led to rapid increase in the demand for secondary batteries as energy sources. Among such secondary batteries, lithium secondary batteries having high energy density and voltage, long lifespan and low self-discharge are commercially available and widely used.
The lithium secondary batteries generally use lithium-containing cobalt oxide (LiCoO2) as an active material. Also, the use of lithium-containing manganese oxides such as LiMnO2 having a layered crystal structure and LiMn2O4 having a spinel crystal structure and lithium-containing nickel oxide (LiNiO2) has also been considered.
LiCoO2 is currently used owing to superior physical properties such as cycle characteristics, but has disadvantages of low stability and high-cost due to use of cobalt, which suffers from natural resource limitations, and a limitation of mass-use as a power source for electric automobiles. LiNiO2 is unsuitable for practical application to mass-production at a reasonable cost due to many features associated with preparation methods thereof.
On the other hand, lithium-manganese composite oxides such as LiMnO2, LiMn2O4 and the like are advantageous in that they contain Mn, which is an abundant and environmentally friendly raw material, and thus are drawing much attention as a positive electrode active material that can replace LiCoO2. However, such lithium manganese composite oxides also have disadvantages such as poor cycle characteristics and the like.
First, LiMnO2 has disadvantages such as a low initial capacity and the like. In particular, LiMnO2 requires dozens of charge/discharge cycles until a constant capacity is reached. In addition, disadvantageously, capacity reduction of LiMn2O4 becomes serious with increasing number of cycles, and in particular, at high temperature of 50° C. or more, cycle characteristics are rapidly deteriorated due to electrolyte decomposition, manganese dissolution and the like.
Meanwhile, lithium-containing manganese oxides include Li2MnO3 in addition to LiMnO2 and LiMn2O4. Since Li2MnO3 is electrochemically inactive in spite of excellent structural stability, it cannot be used as a positive electrode active material of secondary batteries. Therefore, some prior technologies suggest use of a solid solution of Li2MnO3 and LiMO2 (M=Co, Ni, Ni0.5Mn0.5, Mn) as a positive electrode active material.
Advantageously, such a positive electrode active material including Li2MnO3 is very cheap because it contains a great amount of Mn, has high capacity at high pressure and is stable. However, transition occurs from the layered structure to the spinel structure after an activation area in a broad range of 4.4 to 4.6V, thus resulting in weak contact between domains and serious structural change, limiting improvement in electrical properties.
In addition, such an excess manganese-containing positive electrode active material exhibits electrochemical activity because lithium and oxygen are isolated from the crystal structure at a high voltage of 4.3V to 4.5V. For this reason, to offer high capacity, operation is conducted at a high voltage. In this regard, an area which is inactivated during the initial activation process continues to be activated as cycles proceed, disadvantageously causing side-reaction of oxygen originating from the positive electrode active material with the electrolyte and generating a great amount of gas.
In particular, since pouch batteries, unlike rectangular and circular batteries, have a difficulty of maintaining their outer shapes with a predetermined force, pouches may be swollen and thus vented by the generated gas and the gas may remain trapped between electrodes, which interferes with uniform and smooth reactions of the electrodes. In addition, when gases are generated during cycles, gases trapped between electrodes disturb movement of Li ions and the disturbed Li ions are deposited on the surface of the negative electrode, resulting in Li plating which affects resistance increase and deterioration. Furthermore, irreversible Li ions are lost due to Li plating, irreversible capacity is increased during discharge and efficiency is thus decreased. In addition, the trapped gas continuously suppresses movement of Li ions at the initial cycle as well as the subsequent cycles, thus disadvantageously intensifying Li plating and greatly affecting lifespan reduction of batteries.
Accordingly, there is an urgent need for technologies to overcome these problems.